PRESQUE ISLE, Maine — Crops experts with the University of Maine said Thursday afternoon they are “cautiously optimistic” that they can keep the fungal plant disease late blight from ruining Aroostook County’s potato crop.

Steve Johnson, crops specialist for the UMaine Cooperative Extension, said Thursday that while blight has been found in fields throughout The County, the extent of the disease is not as bad as they originally feared.

“It has appeared [in commercial crops],” he confirmed Thursday, “but we expected that we’d see more and in larger quantities. That has not happened, and we are very pleased about it.”

Late blight, which is caused by the fungus Phytophthora infestans, infects plants in the nightshade family such as potatoes and tomatoes and is most destructive during cool, moist weather. The plant’s leaves form black lesions, affected potatoes rot in the soil, and tubers appearing hardy enough to harvest usually rot in storage. The disease can destroy an entire crop rapidly because the associated fungus produces numerous spores that can be carried long distances through the air or transported by infected plants. Late blight was the cause of the Irish potato famine in the 1840s.

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According to the Cooperative Extension Service, late blight has been found on potatoes at one location in the Fort Kent area, one location in the Fort Fairfield area and one location in the Houlton area.

“We have also seen it in Easton and Hodgdon,” said Johnson, “but it has not been in droves at all.”

In other parts of the state, late blight has been found in potatoes in the Old Town area and in potatoes and tomatoes in the Dresden area. Several late blight finds have been made in New Brunswick and one was made at a roadside stand in southern Maine.

Fields are being sprayed with protective fungicides, according to the Cooperative Extension.

At the end of June, agriculture experts across Maine urged home gardeners to check their tomato and potato plants for symptoms of late blight. Big-box stores in Maine had sold infected tomato plants brought in from the southern United States earlier in the season, putting even home gardens at risk, according to earlier news accounts.

Symptoms of the disease are the same in both kinds of plants, according to the Extension. Symptoms begin as wet, light-green lesions, which often appear at the edges of leaves. Within three to seven days of infection, the lesions become brown or black. There also may be a white fungal growth on the undersides of the leaves in humid weather.

Home gardeners are advised to pull all diseased plants and place them in a plastic bag, which should be sealed before discarding. The Cooperative Extension stresses that diseased plants should not be placed into a compost pile or left in the garden.

Johnson said the other threat to potato plants — pests — have not been a significant problem this year.

Cooperative Extension field scouts found trace numbers of potato aphids in fields that were not treated with a systemic material at the time of planting. Potato flea beetles were found in some fields, and European corn borer moths also have been documented.

Still, Johnson said, “the potato crop looks good.”

“If we can get by with some relatively good weather, I think everything will be OK,” he continued. “I think we will be able to keep it in check.”

Officials with the Maine Potato Board were conducting the annual Legislative Tour of the region and were unavailable for comment Thursday.